This invention relates to a nuclear reactor of the type comprising control rods designed in the form of so-called "pin clusters".
In more exact terms, this invention is more especially concerned with a device for guiding nuclear reactor control rods consisting of pin clusters.
A prior art control rod of the pin-cluster type is shown in perspective in FIG. 1 and is constituted by a plurality of absorber pins such as the pin 2 which are disposed in parallel relation and located with a high degree of accuracy. The material employed for the fabrication of these pins must clearly be capable of absorbing the neutrons which take part in the nuclear reaction. The absorber pins 2 are rigidly fixed to the guide stem 4 by means of a so-called "spider" which is constituted by a plurality of radial ribs such as the rib 6. Said ribs are rigidly fixed on the one hand to the guide stem 4 and on the other hand to the upper extremities of each absorbor pin 2. In order to adjust the reactivity of the reactor, that is to say in order to set the neutron flux within the reactor core at a given level, the absorber pins 2 are inserted to a greater or lesser depth within channels formed in the fuel assemblies of the reactor core. This operation is obtained by producing action on the guide stem 4.
It is apparent that the absorber pins 2 have a length which is substantially equal to the height of the reactor core. Furthermore, said pins are attached to the guide stem only at their upper extremities and said absorber pins must be completely withdrawn from the fuel element channels in order to obtain maximum reactivity. It is therefore clearly necessary to guide these absorber pins 2 outside the fuel assemblies in order that they can be readily inserted into said channels. Moreover, the absorber pins are subjected to the action of the pressurized coolant which circulates in the upper portion of the reactor, that is to say in the zone in which the absorber pins are located when they are withdrawn from the fuel assemblies. It is therefore also necessary to protect the pins against the effects of this coolant flow.
The method which is usually adopted for guiding each control rod consists in placing the absorber-pin cluster together with the guide stem inside vertical wrapper tubes of square cross-section which are fitted with internal perforated guide plates having lower extensions in the form of braced and split guide tubes. Said guide tubes (corresponding in number to the absorber pins in the control rod) are slidably mounted in the bottom plate which serves as a bearing member for the fuel assemblies. The bottom plate is rigidly fixed to the top plate by means of spacer columns which are pierced by lateral openings in order to permit the flow of coolant.
This design solution calls for highly accurate positioning of the split guide tubes in order to ensure that the absorber pins are maintained in accurate positional relation. Moreover, this structure is highly sensitive to vibrations.